The present invention relates to a semiconductor light emitting device having a LED (light-emitting diode), and a display device using the same.
A conventional semiconductor light emitting device is shown in FIG. 10. In this semiconductor light emitting device 100, a LED chip 101 is placed on the top of a first lead 102, and one electrode of the LED chip 101 is electrically connected to the first lead 102. The other electrode of the LED chip 101 is electrically connected to a second lead 104 via a gold wire 103. The LED chip 101 and top portions of the first and second leads 102, 104 are sealed with resin 105 which is transparent to a light from the LED chip 101. The LED chip 101 is structured, though not illustrated, to incorporate p-n junction formed by direct joining of a p-type layer and a n-type layer, or double hetero-junction formed by insertion of an active layer between the p-type layer and the n-type layer. The p-n junction portion or the active layer emits light when a forward direction voltage is applied between the p-type layer and the n-type layer.
When an AC voltage is applied to the semiconductor light emitting device 100, a current passes through the LED chip 101 only when the voltage is in forward direction because of rectification of the LED chip.
However, the semiconductor light emitting device 100 has a weakness for a reverse direction voltage, so that the LED chip 101 breaks if reverse direction voltage is excessively generated in AC driving. In the process of handling a substrate of the LED chip, the LED chip 101 also breaks if applied a static electric voltage in reverse direction.
In order to overcome this weakness, a semiconductor light emitting device as shown in FIG. 11 has been proposed. The semiconductor light emitting device 110 has a first lead 111, a second lead 112, a Zener diode chip 115 and a LED chip 116. The Zener diode chip 115 is die-bonded to a concave portion provided on the top of the first lead 111. The LED chip 116 has a p-side electrode and a n-side electrode formed on one surface thereof, and the LED chip 116 is facedown-bonded to the surface of the Zener diode chip 115. The Zener diode chip 115 and the LED chip 116 are parallel-connected in opposite directions to each other. These parallel-connected Zener diode chip 115 and LED chip 116 are connected to the first lead 111 and also to the second lead 112 via a gold wire 114. The Zener diode chip 115, the LED chip 116, and top portions of the first and the second leads 111, 112 are sealed with resin 117, by which the semiconductor light emitting device 110 is constituted.
When an AC voltage is applied to the semiconductor light emitting device 110 and when the voltage is in forward direction, a current passes through the LED chip 116 and the LED chip 116 emits light while the Zener diode chip 115 is in OFF state till the voltage reaches a certain level. On the other hand, when the voltage is in reverse direction, the Zener diode chip 115 is turned on so that high voltage is not applied to the LED chip 116. Accordingly, the Zener diode chip 115 prevents the LED chip 116 from breaking due to reverse voltage.
A display device 120 using the conventional semiconductor light emitting device 110 is shown in FIG. 12. The display device 120 is a dynamic driving display device, in which a plurality of leads are provided in a vertical direction and a horizontal direction. On junctions of these leads, semiconductor light emitting devices are disposed in the form of matrix and connected. The semiconductor light emitting devices are made up of LEDij and Zener diode Zij (i=1 to 3, j=1 to 3). A transistor TRk (k=1 to 6) is provided to each one end of the leads. Those transistors TRk are each controlled so as to light a desired semiconductor light emitting device LEDij, by which images are displayed on the display device 120.
It is noted that the display device 120 is a full color semiconductor light emitting device having red LEDil (i=1 to 3), green LEDi2 (i=1 to 3), and blue LEDi3 (i=1 to 3), where the forward direction threshold voltage of the red LEDil (i=1 to 3) is 2.1V, and the forward direction threshold voltage of the green LEDi2 (i=1 to 3) and the blue LEDi3 (i=1 to 3) is 3.5V.
However, the display device 120 has a drawback that when transistors TR2 and TR5 are turned on to light LED22, a leakage current is passed through Zener diode Z11, as a result of which not only LED 22 but also LED21 and LED12 are slightly lit up, as shown with an arrow A.
Further, because the forward direction threshold voltage of the red LEDil is 2.1V, when the display device 120 is driven at 3.5V or more corresponding to the forward direction threshold voltage of green LEDi2 (i=1 to 3) and blue LEDi3 (i=1 to 3), red LEDil (i=1 to 3) suffers from high remaining voltage in its driver IC (integrated circuit). This imposes a load on the constant current driver IC of the red LEDi1 (i=1 to 3) and raises temperature thereof, causing lower reliability of the driver IC and lower reliability of the display device 120 as well.
Accordingly, a first object of the present invention is to provide a semiconductor light emitting device which is not damaged by a reverse direction voltage at the time of AC driving and free from malfunction due to a leakage current caused by matrix connection.
A second object of the present invention is to provide a highly-reliable semiconductor light emitting device in which when LEDs with different forward direction threshold voltages are connected in parallel so that a large load is not imposed on a constant current driver IC driving a specified LED and therefore the specified constant current driver IC is free from temperature rise.
In order to accomplish the above objects, the present invention provides a semiconductor light emitting device comprising a LED and a protection circuit made up of Zener diodes connected in series in opposite directions to each other, wherein the LED and the protection circuit are connected in parallel.
According to the semiconductor light emitting device of this invention, when a forward direction voltage below the breakdown voltage of the protection circuit is applied to the LED, the reverse-direction Zener diode in the protection circuit is turned off and so the current is not passed through the protection circuit, while on the contrary, the current is passed through the LED and the LED emits light. On the other hand, when a reverse direction voltage below the breakdown voltage of the protection circuit is applied to the LED, the forward direction Zener diode in the protection circuit is turned off and so the current is not passed through the protection circuit, while at the same time, the LED is turned off and therefore the current is not passed through the LED either.
When a forward direction voltage above the breakdown voltage of the protection circuit is applied to the LED, the reverse direction Zener diode in the protection circuit breaks down and so the current is passed through the protection circuit. When a reverse direction voltage above the breakdown voltage of the protection circuit is applied to the LED, the forward direction Zener diode in the protection circuit breaks down and so the current is also passed through the protection circuit. In other words, it can be said that when a voltage above the breakdown voltage of the protection circuit is applied in either forward or reverse direction to the semiconductor light emitting device, the current is allowed to pass through the protection circuit in the direction of applied voltage, which prevents a high voltage applied to the LED, and as a result the LED is protected from damage or breakage. It is noted that in the protection circuit, a total voltage obtained by adding a Zener voltage of the reverse direction Zener diode and a forward direction threshold voltage of the forward direction Zener diode is set to be larger than an operational voltage of the LED.
According to one embodiment of the present invention, the LED is made from a nitride compound semiconductor.
According to this embodiment, the LED made from the nitride compound semiconductor is higher in operational voltage than other LEDs made from, for example, AlGaInP (Aluminum gallium indium phosphorous). Accordingly, when the semiconductor light emitting device is driven with AC, there is possibility that a reverse-direction voltage is excessively applied. However, even though such excessive reverse-direction voltage is applied, the protection circuit breaks down and let the current pass through the protection circuit, by which the LED made from a nitride compound semiconductor can be protected from high voltage applied in reverse direction.
In one embodiment of the present invention, the protection circuit is formed in one silicon chip.
According to this embodiment, the protection circuit made up of a plurality of the Zener diodes is formed in one silicon chip, which makes it possible to down-size the semiconductor light emitting device.
In one embodiment of the present invention, at least one LED is disposed on the silicon chip.
According to this embodiment, the LED is disposed on the silicon chip in which the protection circuit is formed. Therefore, it is possible to down-size the semiconductor light emitting device since at least one LED is disposed on the silicon chip.
The present invention also provides a semiconductor light emitting device in which plural kinds of LEDs different in forward direction threshold voltages, comprising: at least one voltage compensating diode which is connected in series to the LED having a lower forward direction threshold voltage.
According to the semiconductor light emitting device of the present invention, in the case where a plural kinds of LEDs having different forward direction threshold voltages are connected in parallel and driven at a constant voltage, a voltage compensating diode is connected in series to the LED having a lower forward direction threshold voltage so as to compensate for difference in threshold voltage among the LEDs. As a result, the apparent forward direction threshold voltage of the LED with lower forward direction threshold voltage becomes approximately equal to the forward direction threshold voltage of the LED with higher forward direction threshold voltage. Therefore, the remaining voltage of the driver IC in the LED having a lower forward direction threshold voltage becomes approximately equal to that in the LED having a higher forward direction threshold voltage as well. This decreases a load on the constant current driver IC for driving the semiconductor light emitting device and therefore implements stable performance of the semiconductor light emitting device.
In one embodiment of the present invention, the voltage compensating diode is formed in one silicon chip.
According to this embodiment, it is possible to down-size the semiconductor light emitting device since the voltage compensating diode is formed in one silicon chip.
In one embodiment of the present invention, the LED is connected in parallel to one other LED having a forward direction threshold voltage different from that of the LED; and at least one voltage compensating diode is connected in series to either one of the LED and the other LED which one has a lower forward direction threshold voltage.
According to this embodiment, a LED having a protection circuit is connected in parallel to one other LED having a forward direction threshold voltage different from that of the LED, and at least one voltage compensating diode is connected in series to a LED having a lower threshold voltage from among the LEDs so that the voltage compensating diode compensates difference in threshold voltage between the LEDs. Consequently, when a plurality of the LEDs having different forward direction threshold voltages are driven at a constant voltage, the LED having the lower forward direction threshold voltage no longer suffers from a high remaining voltage in the driver IC. Therefore, no load is imposed on the constant current driver IC of the semiconductor light emitting device. Thus, performance of the semiconductor light emitting device is stabilized.
The present invention further provides a semiconductor light emitting device, comprising: a first LED emitting at least one of blue light and green light; a is protection circuit made up of Zener diodes connected in series in opposite directions to each other; and a red light emitting circuit made up of a second LED emitting a red light and at least one voltage compensating diode connected in series to the second LED, wherein the first LED, the protection circuit and the red light emitting circuit are connected in parallel.
According to this present invention, the first LED is higher in forward direction threshold voltage than the second LED when the first LED is made from, for example, a nitride compound semiconductor for emitting blue light or red light and the second LED is made from, for example, AlGaInP for emitting a red light. Therefore, the voltage compensating diode is connected in series to the second LED so as to compensate for difference in forward direction threshold voltages between the first LED and the second LED. Accordingly, even when the semiconductor light emitting device is driven at a voltage corresponding to the threshold voltage of the first LED, the second LED no longer suffers from the increased remaining voltage in the driver IC. Therefore, no load is imposed on the constant current driver IC of the semiconductor light emitting device. This stabilizes performance of the semiconductor light emitting device.
In addition, the semiconductor light emitting device has a protection circuit. Therefore, when an AC voltage below the breakdown voltage of the protection circuit is applied to the semiconductor light emitting device, a forward current passes through the first LED and the second LED to emit lights, and a reverse current does not passe through the semiconductor light emitting device. Further, even when an AC voltage above the breakdown voltage of the protection circuit is applied to the semiconductor light emitting device, whether in forward direction or reverse direction, a current is allowed to pass through the protection circuit. Thereby, the first LED and the second LED are protected. This stabilizes performance of the semiconductor light emitting device and makes the LEDs proof against damage or breakage.
In one embodiment of the present invention, the voltage compensating diode and the protection circuit are formed in one silicon chip.
According to this embodiment, it is possible to down-size the semiconductor light emitting device since the voltage compensating diode and the protection circuit are formed in one silicon chip.
In one embodiment of the present invention, the Zener diodes are connected in an anode common state or a cathode common state.
According to this embodiment, the LED to be connected to the protection circuit need not have a limited polarity.
In one embodiment of the present invention, a dynamic driving display device uses the above-stated semiconductor light emitting device.
According to this embodiment, the dynamic driving display device includes the semiconductor light emitting device having the protection circuit provided with a forward direction Zener diode. Therefore, the forward direction Zener diode intercepts a leakage current in reverse direction. As a result, the display device is free from indication errors. Further, the voltage compensating diode compensates a shortage of the forward direction threshold voltage in the LED. As a result, the forward direction threshold voltage of the LED becomes apparently equal to the higher forward direction threshold voltage of the other LED. Therefore, this prevents a remaining voltage in the driver IC of the semiconductor light emitting device from being increased, and stabilizes performance of the display device as well as performance of the semiconductor light emitting device.